3.1. VLBI and GPS analysis with independent software packages
In a first attempt, the complete CONT11 VLBI data set was analysed with the Calc/Solve software [16] following a standard network analysis approach, as described in [17]. Even the previously mentioned 'problematic' stations (section 3) were included in the analysis. The hydrogen maser at Wettzell (Germany) was used as reference clock for the network and relative clock estimates w.r.t. this reference were estimated for all stations as daily second order polynomials with additional continuous spline corrections with a temporal resolution of 20 min. Since the original data correlation and analysis was done in daily batches, the resulting clock estimates were not continuous over day boundaries. To generate continuous series of clock estimates, the daily solutions were fitted together using overlapping estimates, thus removing potential discontinuities at day boundaries.
In parallel, GPS data recorded with the co-located GPS receivers at the CONT11 stations, all connected to the same frequency standard as the corresponding co-located VLBI-system, were analysed with the NRCAN-PPP software [18]. The analysis procedure is described in [17] and gave GPS clock estimates with 60 s temporal resolution for all analysed stations for the whole CONT11 campaign. These time series were differenced with respect to the solution for the GPS receiver at Wettzell (WTZZ) in order to produce time series of relative clock estimates on the network baselines corresponding to the VLBI solution.
The individual time series of clock differences derived from VLBI and GPS were then analysed and modified Allan deviation (MDEV, [19]) values were derived for each baseline in order to estimate the frequency link stabilities. The results show that VLBI and GPS perform similarly with best case one day instabilities on the order of 10−15 [17]. Figure 3 depicts as an example the results for a subset of 5 baselines connecting to Wettzell. GPS and VLBI derived MDEV values are shown in black dots and orange squares, respectively. It is evident that technical problems at Tsukuba affect the stability of the solution. The TSKB receiver lost lock on day 3 of CONT14, thus the GPS-derived MDEV values involving Tsukuba are deteriorated since the technical problems were not accounted for in this first study.